In recent years, miniaturization and weight saving of mobile information terminals, such as cellular phone, notebook-size PC, and PDA have been remarkably advanced. In accordance with such an advance, demands for higher capacity of a battery as a driving power source have been increasing. In order to meet such demands, as one type of new secondary batteries with high output and high energy density, non-aqueous electrolyte secondary batteries which employ a non-aqueous electrolyte and are adapted for charging and discharging byway of transfer of lithium ions between a positive electrode and a negative electrode, have widely been used.
Generally, the positive electrode active material used for the positive electrode of such a non-aqueous electrolyte secondary battery is lithium cobalt oxide LiCoO2, lithium manganese oxide LiMn2O4 having a spinel structure, lithium composite oxide of cobalt-nickel-manganese, lithium composite oxide of aluminum-nickel-manganese, lithium composite oxide of aluminum-nickel-cobalt, and the like. The negative electrode active material of the negative electrode used includes carbon materials such as graphite, materials to be alloyed with lithium, such as Si and Sn, and the like.
In recent years, because of advancement of amusement function such as moving image reproduction and game function by mobile information terminal, electric power consumption tends to rise more steadily. As a result, demands for further higher capacity and higher function in a battery have been increasing. In order to further increase capacity of non-aqueous electrolyte secondary battery, it is thought to charge the non-aqueous electrolyte secondary battery to a high voltage and to increase the filling density of the positive electrode active material and the negative electrode active material to be filled into the non-aqueous electrolyte secondary battery.
In a case where the non-aqueous electrolyte secondary battery is charged to the high voltage as described above, oxidizing force of the positive electrode active material becomes strong. Further, in a case where the positive electrode active material contains a transition metal having a catalytic property, for example, Co, Fe, Ni, Mn and the like, the non-aqueous electrolyte reacts with the transition metal having catalytic property and decomposes on the surface of positive electrode active material. As a result, charge-discharge cycle characteristics, preservation characteristics and characteristics after sequential charging of the non-aqueous electrolyte secondary battery are greatly deteriorated, gas is generated inside of the battery, and an expansion of the battery is caused. Particularly, the non-aqueous electrolyte secondary battery is further deteriorated under high temperature environment.
In addition, in a case where the filling density of the positive electrode active material and the negative electrode active material to be filled into the non-aqueous electrolyte secondary battery is increased, penetration of the non-aqueous electrolyte into the positive electrode and the negative electrode is not sufficient and an adequate charge-discharge reaction is not attained, so that charge-discharge characteristics are deteriorated. Further, disproportion of the charge-discharge reaction occurs, so that one part of the non-aqueous electrolyte secondary battery is charged to high voltage. As a result, there exists the same problem as in the case of charging the non-aqueous electrolyte secondary battery to the high voltage.
It has been proposed to use a positive electrode active material wherein rare-earth oxides such as La2O3 are contained in complex oxides including Li and Ni or a positive electrode active material wherein rare-earth oxide particle such as La2O3 is adhered on the surface of complex oxide particle including Li and Ni, for the purpose of restricting a reaction between the positive electrode active material and a non-aqueous electrolyte in the case of excessive charging (See, for example, Patent Document 1, JP 2005-196992(A)).
However, even if the positive electrode active material of patent document 1 is used, when the non-aqueous electrolyte secondary battery is charged to the high voltage, the reaction between the positive electrode active material and the non-aqueous electrolyte still occurs. Particularly, under high temperature environment, the charge-discharge cycle characteristics, the preservation characteristics and the characteristics after sequential charging of the non-aqueous electrolyte secondary battery are greatly deteriorated, and gas is generated inside of the battery, so that the expansion of the battery is caused.
It has been proposed to use a positive electrode active material containing LixCoO2 and LiyNisCoxMuO2 and having 10-45 wt. % of Li3NisCotMuO2 against a total amount of LixCoO2 and LiyNisCoxMuO2. Further, M in LiyNisCoxMuO2 are at least one element selected from a group of B, Mg, Al and the like and lanthanoid element, and the elements M form solid solution in the positive electrode active material (See, for example, Patent Document 2, JP 3712251(B)).
However, in such a case, when the non-aqueous electrolyte secondary battery is charged to the high voltage, oxidization decomposition of the non-aqueous electrolyte is not fully suppressed. As a result, as to the non-aqueous secondary battery of Patent Document 2, when the non-aqueous electrolyte secondary battery is charged to the high voltage, the charge-discharge cycle characteristics, the preservation characteristics and the characteristics after sequential charging are greatly deteriorated, and gas is generated inside of the battery, so that the expansion of the battery is caused.
Further, there has been proposed a lithium secondary cell positive activator comprising a core containing lithium compound having the certain particle diameter and being coated with a surface-treated layer containing oxide containing coating element such as Ma, Al, Co and the like, and hydroxide, oxyhydroxide, oxycarbonate, and hydroxycarbonate (See, for example, Patent Document 3, JP 2002-158011 (A)).
However, even in Patent document 3, when the non-aqueous electrolyte secondary battery is charged to the high voltage, the positive electrode still reacts with the non-aqueous electrolyte. Particularly, under high temperature environment, the charge-discharge cycle characteristics, the preservation characteristics and the characteristics after sequential charging are greatly deteriorated, and gas is generated in the inside of the battery, so that the expansion of the battery is caused.
It has been proposed to use a positive electrode which contains lithium-cobalt composite oxide and a rare earth compound selected from a group of lanthanum, cerium and neodymium for the purpose of obtaining a non-aqueous electrolyte secondary battery with high charge-discharge capacity and excellent heat stability (See, for example, Patent Document 4, JP 2004-207098 (A)). In patent document 4, the positive electrode is fabricated by mixing lithium-cobalt composite oxide with rare earth compound such as lanthanum oxide, a conductive material and a binder.
However, if the positive electrode is fabricated the same as patent document 4, rare earth compound such as lanthanum oxide is not adequately dispersed nor adhered on the surface of lithium-cobalt composite oxide. Accordingly, a contact property between lithium-cobalt composite oxide and rare earth compound such as lanthanum oxide is deteriorated, so that sufficient effects can not be obtained. As a result, the amount of rare earth compound such as lanthanum oxide to be mixed with lithium-cobalt composite oxide necessitates being large, and therefore, there exists a problem that the proportion of the positive electrode active material in the positive electrode is decreased.
Further, if the positive electrode fabricated in patent document 4 is used, when the charging voltage is increased, a reaction between the positive electrode active material and the non-aqueous electrolyte occurs. As a result, when the non-aqueous electrolyte secondary battery is sequentially charged under high temperature environments, preservation characteristics and charge-discharge cycle characteristics are not sufficiently improved.
It has been proposed to use a positive electrode active material wherein the surface of a lithium transition metal complex oxide having a spinel structure contains at least one element selected from zinc, yttrium, niobium, samarium and neodymium (See, for example, Patent Document 5, JP2005-216651(A)). This positive electrode active material restricts elution of manganese from the lithium transition metal complex oxide, so that a non-aqueous electrolyte secondary battery having superior characteristics under high temperature environment can be obtained.
However, in patent document 5, a compound to be used as the compound of elements, such as zinc, is not specified. Accordingly, when the charging voltage is increased, a reaction between the positive electrode active material and the non-aqueous electrolyte is not fully restricted. As a result, even in patent document 5, when the non-aqueous electrolyte secondary battery is sequentially charged under high temperature environment, the preservation characteristics and the charge-discharge cycle characteristics are not sufficiently improved.
It has been proposed to use a positive electrode active material wherein a lanthanoid element-containing compound is adhered on at least one part of surfaces of a particle containing lithium-manganese composite oxide. (See, for example, Patent Document 6, JP 2005-174616(A)). Patent document 6 describes that lithium-manganese composite oxide is mixed with an oxide of lanthanoid element such as La2O3, Nd2O3, and Sm2O3 and burned at 550° C. or more. By such a way, the lanthanoid element forms solid solution in lithium-manganese composite oxide and the oxide of lanthanoid element is adhered on one part of the surface thereof.
However, in patent document 6, when the charging voltage is increased as mentioned above, a reaction between the positive electrode active material and the non-aqueous electrolyte occurs. As a result, when the non-aqueous electrolyte secondary battery is sequentially charged under high temperature environment sufficient preservation characteristics and charge-discharge cycle characteristics can not be obtained.
It has been proposed to use a positive electrode wherein lanthanum is added to lithium-cobalt compound oxide for the purpose of obtaining a non-aqueous electrolyte secondary battery with high charge-discharge cycle characteristics and excellent preservation characteristics under high temperature (See, for example, Patent Document 7, JP H04-319259(A)). In patent document 7, in the fabrication of lithium-cobalt composite oxide, lanthanum hydroxide is added and the mixture is burned at high temperature, 900° C. Thus, a positive electrode active material wherein the surface of lithium-cobalt compound oxide is covered with compound oxide of lanthanum oxide, lithium and lanthanum and compound oxide of lanthanum and cobalt is fabricated. Here, the additional ratio of lanthanum against cobalt is 1 to 10 mol %.
However, if the positive electrode is fabricated the same as patent document 7, the additional ratio of lanthanum against cobalt is required to be large and lanthanum forms solid solution in the inside of lithium-cobalt compound oxide. Accordingly, characteristics of the positive electrode active material are deteriorated and charge-discharge efficiency is lowered.
Further, even if the surface of lithium-cobalt compound oxide is covered with compound oxide of lanthanum oxide, lithium and lanthanum and compound oxide of lanthanum and cobalt, when the charging voltage is increased as mentioned above, a reaction between the positive electrode active material and the non-aqueous electrolyte is not fully restricted. As a result, when the non-aqueous electrolyte secondary battery is sequentially charged under high temperature environment, the preservation characteristics and the charge-discharge cycle characteristics are not sufficiently improved.
It has been proposed to use a positive electrode active material wherein yttrium oxide or compound oxide of lithium and yttrium is added to LiCoO2 and the additional amount of yttrium against LiCoO2 is within the range of 1 to 10% in mol ration, for the purpose of restricting destruction of crystal structure of the positive electrode active material (See, for example, Patent Document 8, JP H05-6780(A)).
There has been proposed a non-aqueous electrolyte secondary battery wherein lanthanum, yttrium and zinc are adhered on the surface of positive electrode active material of lithium manganese spinel compound and wherein the metal concentration is not less than 0.01 mol % to less than 5 mol % for the purpose of enhancing charge-discharge cycle characteristics thereof (See, for example, Patent Document 9, JP 2001-6678(A)).
However, even in patent documents 8 and 9, when the charging voltage is increased as mentioned above, the reaction between the positive electrode active material and the non-aqueous electrolyte is not fully restricted. As a result, when the non-aqueous electrolyte secondary battery is sequentially charged under high temperature environment, the preservation characteristics and the charge-discharge cycle characteristics and the like are not sufficiently improved.